It must a puzzel for small children to not see do they make sense of you not being there..Cuba they would understand..
fact is their primary caregiver will show this behaviour often and they minds will blend this into their thinking..when they are older and can choose to visit is something to look forward to.but the seeds have to be sown now,
Its past time that your husband spoke with his son...could he write and say that he may see family counsellors to arrange visits..or could between them set up a visit?

these visits could be on neutral ground all legal and outwith the DIL 's power.For the childrens sake something..things are so bad what would a lawyer add to the unhappiness?
poor son..what a life for him.